This invention relates to apparatus for mixing and dispersing gas in the form of fine bubbles in a body of liquid in a tank by rotating an impeller to pull the gas and liquid into a mixing zone below a submerged shroud where the bubbles are formed and dispersed in an upward radial flow pattern.
The invention can be used in various types of aeration apparatus, such as, to add air to sewage, or remove dissolved oxygen from water by mixing an inert gas with the water to displace the oxygen.
The apparatus also can be used in flotation processes in which solid particles in a slurry, or immiscible liquid droplets in an emulsion, are separated from the main body of the liquid. The small bubbles selectively attach themselves to the particles or droplets to be suspended, and provide buoyancy to raise them to the surface of the liquid. The material to be separated is taken from the surface of the tank in the form of a froth. Chemical reagents can be added to the liquid to enhance film-forming and bubble adherence to improve separation efficiency. Reagents that induce a froth are called "frothers". Those that assist in the selective separation of one solid from another in a liquid are called "depressers", "deflocculating agents" and "collectors", depending on the specific function performed by the reagent.
A good discussion of mixing apparatus on which the present invention is an improvement is in Chemical Engineering, June 8, 1964, pp 165-220.
The following U.S. patents also describe flotation apparatus on which the present invention is an improvement:
953,746 Hoover 1,976,956 MacLean 2,274,658 Booth 2,494,602 Wright 2,626,052 Carbonnier 2,875,897 Booth 3,393,802 Logue et al 3,393,803 Daman et al 3,647,069 Bailey 3,775,311 Mook et al
One prior art flotation apparatus, on which the present invention is an improvement, includes an upright draft tube extending into a body of liquid contained in a flotation cell or tank, and an inverted bowl-shaped hood, or shroud, below the draft tube. The shroud is substantially imperforate, except for a series of radially extending notches formed at spaced apart intervals around the inner periphery (point of maximum elevation) of the shroud adjacent the draft tube. An upright rotary shaft extends down through the draft tube and rotates an impeller located under the shroud. The space under the shroud forms a mixing zone where gas and liquid are subjected to turbulence by the impeller blades. The action of the impeller forms small bubbles which flow outwardly from under the hood through the notches around the top of the hood. The bubbles circulate upwardly in the liquid and attach themselves to material to be removed by flotation. The configuration of the shroud also causes the gas-liquid mixture to be driven down toward the bottom of the tank. This flow pattern tends to sweep the bottom clean of solids and elevate them to a point where they attach themselves to the bubbles and are floated away.
One disadvantage of this prior art mixing apparatus is that the impeller cannot be set deep enough in large tanks to create sufficient circulation to sweep the bottom clean and still produce the necessary surface flow pattern for the air bubbles to effectively remove material by flotation. This prior art unit also has an undesirable tendency to generate foam which flows in a rotary pattern and stagnates around the draft tube. The rotary flow pattern tends to collect material to be floated in the corners of the cell. The stagnation causes a build up or collection of foam in the center of the cell where the foam either dissipates or is "folded under" by the flow pattern. Therefore, even though contaminants are floated to the surface, a good part of them are reentrained in the liquid and have to be floated again. The present invention avoids these problems by generating a flow pattern of air bubbles in a radial direction outwardly from the mixing apparatus toward the edges of the cell where skimmers can remove the material floated to the surface.
Flotation processes commonly use several side-by-side tanks or cells through which the treated liquid flows serially. It is common to have a liquid level gradient from cell to cell, with the level of liquid in the cell nearest the inlet being the highest, and the levels in each cell thereafter being progressively lower. The level in each cell is commonly set by adjusting the elevation of weirs on opposite sides of each cell. The present invention provides a convenient means for adjusting the gas-to-liquid ratio in each cell which, in turn, provides a good way of complementing the use of adjustable weirs to adjust the liquid level gradient from cell to cell.